英语序数词前面不一定都要加the，应视情况而定。当序数词单纯地表示“第几”的时候就加the，当序数词用于“又一”的意思时则用a，当序数词前有人称代词时，不加the。

序数词什么情况下不加the

1．当序数词前有形容词性物主代词或名词所有格修饰时,序数词前不用the.

Mother was my first teacher in my life.妈妈是我生命中的第一个老师.

Tom is Lily's third boyfriend.汤姆是莉莉的第三个男朋友.

2．当表达分数时,序数词前不用the.

One fifth of the students here are from the country.这儿1/5的学生来自农村.

Three fourths of the students here are from the country.这儿的3/4的学生来自农村.

3．当表达“又一,再一”时,序数词前不用the,但可加不定冠词.

Please give me a second chance.请再给我一次机会.

The poor woman had a third baby.那个可怜的妇女又生了第三个宝宝.

4．当表达“年月日”时,尤其在朗读时,序数词前要加the.

He was born on June 6th,1974.他出生在1974年6月6日.

5．当序数词在句中作状语时,序数词前不用the.

Who got there second?谁第二个到那儿的?

I finished the work first.我最先完成那项工作.

6.当表达考试（赛跑等）中获得的名次时,序数词前不用the.

She stands first in her class.她在班上成绩排名第一.